I'm not saying I don't understand the basis on which the current system determines who gets to plead diminished responsibility and who doesn't (and whose pleas are ultimately likely to be successful - interestingly many murderers with schizophrenia are not deemed to have diminished responsibility). But I don't think it's beyond question and I think many of the assumptions it's based on are false. Like when you say that there's still some element of insight and control over your reactions even with severe ADHD (the position taken by most courts), simply because people aren't going around reacting to every impulse or annoyance, I really cannot agree. To me this just seems like a lack of understanding as to how loss of control comes about in the first place and what it means to live with and try to manage extreme levels of impulsiveness (I do not have this type of ADHD myself but I have read a lot about it and about similar conditions).
I don't think it follows that because someone has insight and control most of the time that they therefore have it all of the time and I can't see how that could ever even be knowable for every individual in every instance. I think the evidence actually points to the fact that some people with severe Hyperactive/Impulsive ADHD can briefly and totally lose both insight and control over their actions even when they're making every effort to ensure that doesn't happen. Often because the amount of effort it takes to suppress constant, unwanted impulses is immense, especially if undiagnosed and unmedicated, and sometimes exertion exhaustion sets in and the person becomes temporarily unable to continue managing them. In an ideal world, you would remove yourself from the company of other people or situations in which you might get into trouble if you were aware you were reaching that point but we don't live in an ideal world - some sufferers also have massive impairments in self-awareness and forward planning abilities and sometimes even when they get everything right, real life conspires against them. Jails are full of people with head injuries and ADHD for this reason. I'm sure some of them at least would have avoided being there had they genuinely been able to control their behaviour at all times, and the fact that crime and recidivism rates drop so dramatically when ADHD is identified and treated supports that.
It also seems that if an autistic person is violent to others during a meltdown they are unlikely to be able to plead diminished responsibility even though the evidence emerging from the autistic community suggests that most individuals are not in control of their behaviour in these moments either.
Ultimately, the system is unlikely to change because we don't have the resources to treat every violent individual as a patient and nor is it safe to let them all go free. But I don't think the fact that schizophrenia is more likely to be accepted as grounds for diminished responsibility is any sort of definitive proof that people with severe ADHD have either insight or full control over their behaviour at all times.
Also, as noted above, most schizophrenics do not go off and beat someone up randomly - strangers are rarely their victims and there's usually background to the situation like existing family tensions, untaken medication and drug and alcohol use. According to this article from Reuters "Pooling data from seven studies in four countries, researchers found that the odds of a person with schizophrenia killing a stranger were 1 in 14 million people per year.").. It's almost always people known to them who they kill. Presumably they were in control of their actions until they very suddenly weren't, or else their victims would have removed themselves from their presence before it was too late. Loss of control seems to happen very quickly in all sorts of conditions that affect the brain and clearly cannot always be easily anticipated.
And what does something changing your personality have to do with anything? ADHD cannot change your personality because you have it from birth - there was never a time when the sufferer didn't have it so you can't really say what the person would have been like without it in the way you can with schizophrenia which is very rarely developed before adolescence at the earliest. Unless you look at the difference in behaviour before and after ADHD medication in someone for whom medication is effective (it obviously isn't for many people). This can often be quite extreme and in my opinion can be indicative of which aspects of a person's ADHD may truly be within their control. However, it may work on some symptoms but not others, or a person may find it effective but not be able to tolerate it which complicates things.